The object of the present invention is a non-evaporable ternary gettering alloy and its method of use for the sorption of water, water vapour and other gases.
So called getters or sorbers are well known in the art and are described in numerous patents, such as for example in U.S. Pat. Nos. 3,203,901 and 4,071,335. In an article appearing in the journal "Electrochemical Technology" Vol. 4, n. 5-6, May/June 1966, pag. 211-215, A. Pebler and A. Gulbransen have also described the use of the intermetallic compound ZrV.sub.2 for the sorption of hydrogen, but this material in the form of a powder is pyrophoric.
Ternary alloys of Zr, V and Fe have been described by D. Shattiel, I. Jacob and D. Davidov in J. Less-Common Metals 53 (1977) 117-131 for the reversible storage of hydrogen at atmospheric and higher pressures. Such alloys have the composition Zr(Fe.sub.x V.sub.1-x).sub.2 where 0.ltoreq.x.ltoreq.1.
Non-evaporable ternary gettering alloys have also been described for example in British Pat. No. 1,370,208 where there is specifically mentioned the alloy based on zirconium, Zr-Ti-Ni and its usefulness when it is wished to stoichiometrically sorb humidity or water vapour, as well as other gases, without the release of hydrogen.
The desire to have a gettering material capable of sorbing water without releasing hydrogen is particularly felt in the nuclear energy production industry, as the fuel rod sheathing most commonly used is a material based on zirconium, which in the presence of hydrogen and/or water is subject to cracking. Water is released mainly from the sintered fuel pellets, of a ceramic nature, usually UO.sub.2, which are contained within the fuel element. Even though these fuel pellets are subjected to a heat treatment in order to remove water vapour, during their use in a nuclear reactor they still release water vapour.
The presence of water vapour is also undesirable in other cases, for example within the envelopes of incandescent filament lamps where the water vapour takes part in the well known "water cycle", due to which there is a continuous transfer of tungsten from the filament to the walls of the lamp with a subsequent blackening or darkening of the glass and premature erosion of the filament and therefore lamp destruction.
Also in high pressure discharge lamps it is necessary to maintain a low level of hydrogen and other gases within their jacket.
The known ternary alloy Zr-Ti-Ni, as known from the above mentioned British patent specification, must be used at a temperature of 350.degree. C. or more, to ensure that during the sorption of water and water vapour there is no release of hydrogen. That temperature corresponds approximately to that at which the getter device finds itself when used for example in boiling water reactors or pressurized water reactors ("BWR" or "PWR" reactors). However during the starting phases of a new reactor and also when the reactor is working at low power, the temperature may be considerably lower than 350.degree. C. Therefore during these periods of operation there is a danger of a harmful release of hydrogen.
It would therefore be very useful to have available a getter material able to sorb water and water vapour, without release of hydrogen, at temperatures less than 350.degree. C.
Furthermore it would be useful to have a gettering material having a high sorption velocity able, therefore, to sorb as quickly as possible all the water and all the water vapour within the ambient in which the getter is used.
In other traditional getter device applications, for example in certain electron tubes, it is possible to activate the getter at a relatively high temperature, then using the getter at a pre-determined lower temperature for sorbing certain gases.
In particular there may be required a getter capable of working at ambient temperatures as it may not be possible to supply energy in order to make the getter work in a continuous way at higher temperatures, or the device may not be able to support these higher temperatures.
The activation of these getters takes place by means of a thermal treatment for a certain length of time. Normally this thermal treatment takes place at temperatures between 700.degree. and 1000.degree. C. for a time of several tens of seconds up to several minutes. The activation process apparently allows the surface passivated layers of the gettering material to diffuse into the individual particles of the material thus providing a clean surface capable of sorbing gas.
If, however, it is wished to activate the known getter at lower temperatures, the time necessary for this diffusion becomes prohibitively long, of the order of several hours. Nevertheless in some cases it is undesirable or even impossible to allow the getter device to reach temperatures of the order of 700.degree. to 1000.degree. C., for which reason it is difficult to activate the getter device.